This invention relates to semiconductor processing of substrates, and more particularly to shadow frames used to prevent deposition on the edge and backside of a substrate.
The semiconductor industry has been using single substrate (silicon wafer) processing chambers for some time because the chamber volume can be minimized, contamination of the substrate reduced, process control increased. Therefore, yields are improved. Further, vacuum systems have been developed, such as described in U.S. Pat. No. 4,951,601, that allow several sequential processing steps to be carried out in a plurality of vacuum processing chambers connected to a central transfer chamber, so that several processing steps can be performed on a substrate without its leaving a vacuum environment. This further reduces the possibility of contamination of the substrates.
Recently the interest in providing large glass substrates with active thin film transistors thereon for applications such as active matrix TV and computer displays has been heightened. These large glass substrates require vacuum processing chambers for deposition of thin films. The basic methods and processing chambers, e.g., plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD or xe2x80x9csputteringxe2x80x9d) etch chambers and the like, are similar to those used for depositing layers and patterning thin films on silicon wafers. A practicable system that can perform multiple process steps on glass substrates is disclosed by U.S. Pat. No. 5,512,320, hereby incorporated by reference. However, because of the large size of the glass substrates, several problems have been noted in their handling and processing in vacuum processing chambers.
During processing, the edge and backside of the glass substrate must be protected from deposition. A deposition-masking rectangle or shadow frame is placed about the periphery of the substrate to prevent processing gases or plasma from reaching the edge and backside of the substrate in a CVD chamber for example. The susceptor, with a substrate mounted thereon, can have a shadow frame which will surround and cover several millimeters of the periphery of the front surface of the substrate. This will prevent edge and backside deposition on the substrate. If, however, the shadow frame is not properly centered with respect to the substrate during processing, the amount of shadowing that occurs on each edge of the substrates will be unequal and unacceptable.
In substrate processing in general, and in PVD (sputtering) processing in particular, particulates which are present and are generated in the processing chamber can contaminate and destroy the substrate being processed. When such particulates (also known as xe2x80x9cfreexe2x80x9d particulates) land on the substrate being processed, they contaminate a small area of the substrate which can be discarded if the substrate is die cut into separate chips. However, when a large substrate is intended for subsequent use as a single item (e.g. as a flat panel display), one defect causes the whole unit to be rejected.
The contaminating particulates originate from several sources. Incomplete or defective cleaning of the chamber allows particulates to remain in the chamber and cause contamination. However, even when the processing chamber is clean, contaminants can be and are generated during the sputtering process. One type of contaminating particulate originates from sputter deposited material which has deposited itself on processing chamber surfaces other than the substrate intended for deposition, and subsequentially detaches (peels off or falls off) from the location inside the vacuum processing chamber where it originally had been deposited. These particulates are usually cool, multi-molecular sized specks of sputter deposited material which were hot during the sputtering process, but have since cooled as a result of their contact with surrounding surfaces. However, unlike the hot material being sputter deposited when the cool specks (particulates) land on and are embedded in the substrate, they can create defects which cause rejection of the substrate.
Another source of particulates is electrical arcing between the highly charged (biased) target and its surrounding uncharged (grounded) pieces. Arcing occurs in PVD processing chambers at locations between the edge of the target and surrounding surfaces (usually a shield enclosing the target and protruding into the space adjacent to the target which is known as xe2x80x9cthe dark space ring or groovexe2x80x9d). Arcing between adjacent pieces causes a severe localized temperature spike which in most cases releases molecules of one or both of the materials between which the spark arcs. If the released molecules settle on the substrate, at best, they create a slight but acceptable anomaly in the coating pattern, or at worst when a particulate is a foreign material, the substrate will be contaminated and will have to be rejected.
In one aspect, the invention is directed to a vacuum processing chamber. The chamber has walls defining a cavity for processing a substrate, a substrate support for supporting a substrate being processed in the cavity, a shadow frame for preventing processing of a perimeter portion of the substrate, and a shadow frame support supporting the shadow frame within the cavity. The shadow frame is positionable with a gap between an underside of the shadow frame and an upper surface of the substrate, and the shadow frame support has at least one conductive element insulated from the walls and establishing a conductive path from the shadow frame to outside the cavity.
Implementations of the invention may include the following. The substrate support may include a susceptor having an upper surface to support the substrate, and a plurality of lift pins vertically movable relative to the susceptor to contact the underside of the substrate to support the substrate. A bias voltage source may be coupled to the conductive element and configured to apply a bias voltage to the shadow frame selected to prevent arcing between a portion of the substrate being processed and the shadow frame. The bias voltage source may be configured to apply a second bias voltage to the shadow frame to attract charged particles, released by termination of a plasma within the chamber, to the shadow frame, so that there is a preferential accumulation of such particles on the shadow frame relative to the substrate. A voltage measurement device may be coupled to the conductive element and configured to measure a voltage of the shadow frame. A grounding circuit may couple the conductive element to ground, and it may be configured so as to discharge a first charge accumulated by the shadow frame during processing. The discharge may maintain the shadow frame at a potential sufficient to substantially prevent arcing between the shadow frame and the substrate. The grounding circuit may include a resistor to continuously discharge charge accumulated by the shadow frame, a switch connecting the at least one conductive element to ground, a voltage measurement device configured to close the switch if the charge on the shadow frame exceeds a threshold, or a control system configured to close the switch after a predetermined number of substrates have been processed. The conductive element may be a height-adjustable generally vertical member, which may be adjusted to determine the height of the shadow frame within the cavity. The shadow frame support includes a mounting flange insulated from the conductive element. The height-adjustable member may include an upper member having an at least partially cone-shaped upper end in physical and electrical contact with a mating feature in an underside of the shadow frame and a lower end having a threaded central aperture, a lower member having an externally threaded upper portion in threaded engagement with the threaded central aperture of the upper member, and a lock nut in threaded engagement with the externally threaded upper portion and configured to bear against the lower end of the upper member to lock the upper member relative to the lower member. The support may be movable in a vertical direction to move the substrate between a loading height and a processing height, above the loading height;
In another aspect, the invention is directed to a process for sputter deposition from a sputtering target onto a surface of a substrate positioned in a processing region of a vacuum chamber. In the process, a plasma is generated between the target and the surface of the substrate, material is sputtered from the target onto the surface of the substrate to form sputter deposited material thereupon, the generation of the plasma is terminated as to release particles having a charge, and a bias voltage is applied to a shadow frame sufficient to attract at least a portion of the particles to the shadow frame, whereby the portion of the particles does not contaminate the surface of the substrate.
Implementations of the invention may include the following. The plasma may be moved between first and second positions in the vacuum chamber by moving a magnet in a magnet region of the vacuum chamber between a home position and a remote position at extremes of a range of the magnet. The step of applying a bias voltage may occur while the magnet is substantially at said home position. The sputtering step may be performed with a substantial quantity of argon gas in the vacuum chamber and the bias voltage may be a positive voltage.
In another aspect, the invention is directed to a process for sputter deposition from a sputtering target onto a surface of a substrate positioned in a processing region of a vacuum chamber. In the process a plasma is generated between the target and the surface of the substrate, material is sputtered from the target onto the surface of the substrate to form sputter deposited material thereupon, and the generation of the plasma is terminated. Charge is discharged from a shadow frame at a rate sufficient to maintain the shadow frame at a potential that substantially prevents arcing between the shadow frame and the substrate.
Implementations of the invention may include the following. The charge may be discharged continuously through a resistor. The voltage of the shadow frame may be measured, and the charge may be discharged when the measured voltage exceeds a threshold. The charge may be discharged after a predetermined number of substrates have been processed.
In another aspect, the invention is directed to a vacuum processing chamber. The chamber has walls defining a cavity for processing a substrate within the cavity, a substrate support for supporting the substrate being processed in the cavity, a shadow frame for preventing processing of a perimeter portion of a substrate, and a shadow frame support supporting the shadow frame within the cavity. The shadow frame is positionable with a gap between an underside of the shadow frame and an upper surface of the substrate, and the shadow frame support has a plurality of conductive elements insulated from the walls and establishing at least one conductive path from the shadow frame to outside the cavity in order to discharge charge from the shadow frame at a rate sufficient to prevent a voltage differential from accumulating between the shadow frame and the substrate which would cause arcing therebetween.
Among the advantages of the invention are the following. Arcing between the substrate and the shadow frame may be reduced, thereby decreasing the accumulation of contaminating particles on the substrate.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.